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Tony Hallas’ Cosmic Imaging: Working with RegiStar

May 2011: This indispensable software will greatly assist your image processing.
tony_hallas
In this month’s column, I want to take a basic look at how the software RegiStar works. This program is an image alignment, or registration, program designed so that amateur astronomers can work with their images. Before RegiStar — a product of Auriga Imaging in Vancouver, British Columbia (visit the company website at www.aurigaimaging.com) — trying to align frames in a mosaic of celestial images was a nightmare.

RegiStar has the ability to morph one frame onto another, so it works best when you have a single image (called the base image) of the entire field of view that your individual shots encompass. This single image can be one of lower resolution that you have upsampled (increased in size beyond the point where it retains its quality) to the final size of the mosaic.

As long as RegiStar can see a basic star pattern, it can do the job. The process involves you carefully morphing each individual shot over the base image until all that remains are the high-resolution mosaic components. Here’s the process, step by step:

1  Start by importing your base and mosaic images into RegiStar. (Select “File,” then “Open,” and then choose your images).
To-create-a-mosaic
To create a mosaic, RegiStar requires a background image and however many mosaic images you've chosen to shoot. The background image need not be high-resolution, or even in color, but it must show the entire field of view covered by all the mosaic images. All images: Tony Hallas
2  Click the “Align” icon and a dialog box will open. Under “single source,” RegiStar wants to know which image you want to register. Choose one of your high-resolution mosaic images. Make sure you highlight one when you open this dialogue box so that it will appear in the box.

3  The second box asks which image is to be the “reference image” (for this image group). Here is where you will select the single base image to morph the mosaic pieces onto. You can scroll through the choices; they will be whatever you have open in RegiStar.

4  Next, hit the “Register” button at the bottom of the box. RegiStar will now scan your two files and align the top one to the base. Repeat steps 2 though 4 for each high-resolution mosaic image.

5  When RegiStar finishes, the aligned file will have a funny color. This is just the way the program displays it. You now want to “Crop/Pad” the aligned image. This saves it in a sized format that will allow assembly via layers in Photoshop. Click the last icon button on the right to open the “Crop/Pad” dialogue box. This box allows saving the “Crop/Pad” in a variety of ways: either in proportion to the entire base, or cropped to fit the area it will occupy in the final image. Experiment and see what works best for you. RegiStar also has a help menu.
One-of-the-last-steps
One of the last steps to creating a mosaic is to use RegiStar’s "Crop/Pad" function. (See number 5.) This process allows you to assemble your final image as a stack of layers.
When you return to Photoshop, open the base image and make it the background. Then import each “Crop/Pad” image as a layer on top of it until you have the full mosaic. Notice how the images fit perfectly! Blend them by using the technique I described in last month’s column.

Along the way, I’ve learned a few additional things about RegiStar. Sometimes the program encounters too much noise in the image, which it mistakes for stars. If this happens, try changing the choices for “Noise Compensation” to Level 1.

If you import an RGB image with bad star alignment into RegiStar, the program can separate the image into individual channels. You then can align them in the same way as your other files and recombine them — with perfect star alignment.

RegiStar has become a fundamental building block of image processing. It makes easy work of my most difficult projects. I’m sure it will do the same for you.
In next month’s column, I’ll introduce you to a technique I invented called the Green Pixel Gun.
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